Precoding is a kind of beamforming that supports multi-layer transmission in MIMO radio systems. Simulations have shown that precoding increases the performance of wireless communication systems and it has therefore been included in the standardization of LTE. In precoding, each of the multiple streams are emitted from the transmit antennas at the base station with independent and appropriate weighting per each antenna such that the throughput is maximized between the base station and the User Equipment (UE). The precoding weights may be calculated at the UE and then the UE informs the base station which precoding weights that should be used.
Usually, only a limited number of predefined precoding weights are used, which are stored in a codebook. The codebook is known both at the base station and the UE, so when the UE informs the base station which precoding weights that should be used, the UE only needs to send a number (index) corresponding to the place that the precoding weights have in the codebook. This number is usually called Precoding Matrix Indicator (PMI). The more antennas that are used at the base station, the more precoding weights are needed in the codebook. Future wireless communication system will probably use 8 or more antennas at the base station, which will need a quite large codebook.
There are several issues with a large codebook: the calculation of which precoding weights to use becomes time consuming and energy consuming; many bits must be send from the UE to the base station to inform which precoding weights to use; there is a larger probability that precoding weights corresponding to “un-physical” precoding vectors are chosen. Therefore, there is a need for reducing the size of the codebook by removing these un-physical precoding vectors from a predetermined codebook whenever possible.
Future wireless systems will use reconfigurable antennas at the base station, to increase the capacity of the communication network and save energy. Two important features of reconfigurable antennas are that they can change the pointing direction of the beams and the beam width for an antenna port, and thereby the radiation pattern and the coverage area of the base station.
Different precoding weights will generate precoding beams in different directions. However, with reconfigurable antennas the beams of the antenna elements at the base stations will be changed over time. If the precoding beams that are created by the precoding weights are pointing in another direction than the radiation pattern of the antenna elements, the antennas will become mismatched and very little part of the power supplied to the antennas will be transmitted.